So i was about the upgrade my unity to 4.5.5 and I thought I better check that I can still open the project in a previous 4.5 version as i’m sure uni wouldn’t be updated to the latest.
You can. To force Unity open your project no matter of its version, enable Meta files and Force Text serialization. This way you will enforce Unity opening your project, but you will most likely need to edit some stuff in your project to make it work, especially if you open something that used new features in an old editor, e.g. you will manually need to downport.
Already asked here. Unfortunately it seems you can’t.
However, you can keep different versions of Unity installed (you only need to install them in different locations) as well as different versions of your projects.
When upgrading a project to a newer Unity Version, there is a good chance that it won’t work anymore in older Versions. I don’t know exactly if it is anyhow possible to work on a project from a newer Unity version with an old Unity version.
As far as I have experienced it, this never worked. So the answer to this question is no.